A problem associated with design of various electrochemical cells, particularly alkaline cells, is the tendency of the cell to produce gases as it continues to discharge beyond a certain point, normally around the point of complete exhaustion of the cell's useful capacity. Gas may also be generated when a cell is force discharged as occurs when cells of nominally equal but not absolutely identical capacity are discharged in a series configuration. Gas may also be produced when a cell is charged, particularly if the cell is not specifically designed for use as a rechargeable cell or if an incorrect charger is used. Additives may be included with the anode and cathode or various cell components may be coated to retard such gassing. Nevertheless, electrochemical cells, particularly alkaline cells, are conventionally provided with rupturable diaphragms or membranes within an end cap assembly for the cell's casing. A cell having such rupturable diaphragm is described, for example, in U.S. Pat. No. 3,617,386. Such diaphragms are designed to rupture when gas pressure within the cell exceeds a predetermined level. The end cap assembly may be provided with vent holes for the gas to escape when the diaphragm or membrane is ruptured.
Although such rupturable diaphragms provide the cell with a significant safety feature for releasing gas pressure they do have the disadvantage that corrosive or harmful electrolyte could possibly be carried out the vent passages along with the gas.
Cells can be provided with pressure responsive members within the cell body which rupture or become dislodged causing a break in the electrical pathway within the cell itself, that is, between one of the cell electrodes (anode or cathode) and a corresponding cell terminal (positive or negative terminal). A pressure responsive current interrupt device of this type is described in U.S. Pat. No. 4,035,552. Such pressure responsive current interrupt devices add significantly to the cost of the cell, since they are designed to function while remaining wholly within the cell. They also have the disadvantage of using up a significant amount of space within the cell which could otherwise be used for additional anode or cathode material.
Other types of pressure activated current interrupt devices for cells utilize an external contact ring which electrically connects the cell casing to the peripheral edge of a cell terminal plate at the closed end of the casing. A pressure activated current interrupt device of this type is described in U.S. Pat. No. 4,992,344. The casing in turn is in electrical contact with one of the cell electrodes, typically the cathode in alkaline cells. The body of the terminal is otherwise insulated from the cell casing. When gas pressure builds within the cell the closed end of the casing bulges causing the conductive ring to break its connection with the terminal plate thereby breaking the electrical pathway between electrode and terminal plate to shut down the cell. Although a portion of this type of current interrupt device is external to the cell, the device as a whole has proved to be costly to manufacture and functionally unreliable.
U.S. Pat. No. 2,651,669 describes a cell with a deformable flat diaphragm which is inserted as a separate piece within the open end of the casing for the cell. When pressure builds within the cell the diaphragm is intended to deflect outwardly causing an insulating stud in contact therewith to protrude from a terminal of the cell to break electrical contact with another device. The described design has the disadvantage that the deformable diaphragm must be inserted into the cell casing as a separate piece thereby requiring independent sealing between the diaphragm and the cell casing.
Accordingly, it is desirable to provide the cell with an auxiliary or substitute pressure activated current interrupter device which is readily manufactured, reliable, does not require additional sealing and does not significantly reduce cell capacity. It is desirable to provide a cell with such pressure activated current interrupt device which activates before pressure builds up to a level high enough to cause venting of gas and expulsion of electrolyte from the cell.